1. Field of Invention
The present invention relates to reduction of non-linear cross-talk in low dispersion fiber. More specifically, non-linear cross-talk is reduced by introducing walk-off between transmission channels.
2. Description of Related Art
For high-capacity, long-haul data transmission, two effects pose a limit to the capacity of a Wave Division Multiplexed (WDM) system. First, chromatic dispersion of the transmission fiber limits the bit-rate per channel. Second, non-linear inter-channel cross talk limits the number of WDM channels that can be transmitted simultaneously.
Chromatic dispersion in standard, single-mode fiber (SMF) and, for larger systems, in non-zero, dispersion-shifted fiber (NZDSF), distorts the transmission of high bit rate signals. Dispersion compensating fibers as well as chirped fiber gratings are popular compensation methods for such distortion. Both techniques effectively compensate for the accumulated dispersion of the transmission system, and thus, erase the temporal broadening of a signal pulse.
Chromatic dispersion of the transmission fiber requires dispersion compensation over the optical bandwidth of each channel. However, conventional dispersion compensation schemes, e.g., dispersion compensating fibers or chirped fiber gratings, perform dispersion compensation over the whole bandwidth of the WDM system implementing overall dispersion compensation (ODC). As a result, such schemes compensate for dispersion in the bandwidth between transmission channels.
However, the effect of non-linear cross talk worsens when dispersion compensation between the transmission channels is performed. Chromatic dispersion causes the signals in different WDM channels to travel at different speeds, which reduces the impact of non-linear cross talk due to averaging. It is well known that using fiber with a low chromatic dispersion coefficient D, e.g., less than 1 pico-second per kilometer-nanometer (psec/kmxc2x7nm), for transmission of a WDM signal results in signal distortion due to non-linear inter-channel effects, e.g., four-wave mixing and cross-phase modulation. The lower the dispersion value D, the closer the coupling between different WDM channels and the larger the interfering cross-talk.
Non-linear inter-channel effects can be mainly attributed to two relationships. First, a low dispersion coefficient enhances xe2x80x9cphase matchingxe2x80x9d between channels and causes interfering four-wave mixing products. Second, as the channels propagate with nearly the same velocity, no walk-off between the channels occurs. Walk off is a de-correlation in the transmission velocity of the bits of data in neighboring WDM channels. As a result, interference in consecutive fiber spans always occurs between the same signal bits. Thus, cross-channel interference accumulates more rapidly. Therefore, mitigating the impact of non-linear cross talk is one benefit of chromatic dispersion in optical fiber spans.
As a result, implementation of dispersion compensation techniques causes an increase of the detrimental effects of non-linear cross talk. One way of mitigating the effects of non-linear cross talk is to increase the separation band between transmission channels. As a result, the effects of conventional implementation of chromatic dispersion techniques often must be offset by using a larger frequency separation between the WDM channels.
Alternatively, other conventional dispersion compensating devices perform channel-by-channel compensation (CCC) of dispersion, thereby eliminating dispersion compensation in the bandwidth between transmission channels. Such devices are also more compact than devices compensating the dispersion of a whole transmission band. Operation of such devices can be extended towards a tunable amount of dispersion. Nevertheless, larger frequency separation between the WDM channels is still necessary to mitigate the effects of non-linear cross talk between transmission channels.
Additionally, new fiber types having higher chromatic dispersion coefficients, e.g., greater than 1 psec/kmxc2x7nm, have increased the dispersion in the transmission-band while, at the same time, keeping it low enough to not require dispersion compensation. Such fibers have a zero-dispersion wavelength between 1450 nm (TW-RS, Lucent) and 1510 nm (E-LEAF, Corning). While the capacity of those fibers is sufficiently high for WDM transmission in the C-band, future operation in the S-band presents problems due to the low chromatic dispersion in this S-band wavelength range.
However, the channel count in announced WDM systems is 160 or 240 channels. This high channel count can only be obtained, when the channels are packed very closely, i.e., with a channel separation of 50 GHz or 100 GHz, and transmission bands in addition to the conventional C-band, i.e., 1530-1565 mn, are used. However, future WDM systems will operate in the L-band, i.e., 1570 nm-1620 nm, and S-band, i.e., 1440 nm-1500 nm. Nevertheless, such future systems will face severe restrictions on the usability of the new transmission bands because of the infrastructure of presently deployed fiber system architectures.
To more effectively handle such operation, the present invention provides a method and system for channel-by-channel dispersion compensation that mitigates inter-channel interference. Through transmission simulations, it has now been established that inter-channel cross-talk can be partly reduced by introducing a walk-off between the channels after each amplified transmission span. The present invention reduces inter-channel cross-talk by introducing a walk-off between the channels after each amplified transmission span.
In a first exemplary embodiment of the invention a compensator assemblage comprises a first compensator that is an ODC dispersion compensator, e.g., dispersion compensating fiber, chirped fiber grating, introducing dispersion D and a second compensator that is a channel-by-channel dispersion compensator introducing dispersion xe2x88x92D. While the dispersion D, which is introduced into each channel of an optical link by the first compensator, is annihilated by the second compensator, the walk-off between the channels, introduced by the first compensator, remains. As a result, the effects of non-linear inter-channel interference are mitigated. The first exemplary embodiment is preferably implemented in a WDM network having no need for compensation mechanisms implemented for the purpose of compensating for dispersion.
A second exemplary embodiment is preferably implemented in a WDM network having need for compensation mechanisms implemented for the purpose of compensating for dispersion D, which is produced in the network as a result of dispersive fiber spans used to transmit data. Accordingly, in the second exemplary embodiment of the invention, a first compensator is utilized for the purpose of performing ODC by introducing dispersion xe2x88x92D. However, the amount of dispersion introduced by the ODC is offset by an offset value xcex4. Therefore, the amount of dispersion introduced by the first compensator is xe2x88x92D+xcex4. A second compensator which is a channel-by-channel dispersion compensator introduces a dispersion xe2x88x92xcex4. While the +xcex4, which is introduced into each channel by the first compensator, is annihilated by the second compensator introducing dispersion xe2x88x92xcex4, the walk-off between the channels, introduced by the first compensator, remains along with the dispersion xe2x88x92D provided by the first compensator. As a result, the effects of non-linear inter-channel interference are mitigated while the level of dispersion D in the WDM network is annihilated by the dispersion xe2x88x92D provided by the first compensator.